Recently, hologram recording has been studied actively since the hologram recording enables high-density recording, multi-recording, and the like. As the hologram recording, recording using an amplitude hologram utilizing a change in transmittance of a recording material, recording using a phase hologram utilizing a change in a refractive index and a change in unevenness of a recording material are known.
Among the recording materials used in hologram recording, a hologram recording material whose refractive index changes due to light irradiation (hereinafter may be referred to as “photorefractive material”) has been studied extensively. In particular, an organic photorefractive material has been studied actively because of the ease of forming into any appropriate shape and the ease of regulating a response wavelength.
In the photorefractive material, charge is generated by light irradiation, the generated charge moves to be trapped, an internal electric field is generated consequently, and a refractive index changes due to the Pockels effect caused by the internal electric field. A hologram is formed by the change in refractive index.
However, in the organic photorefractive material, it is required that molecules be aligned so as to express the Pockels effect effectively, which requires an external electric field. The necessity for an external electric field is an important problem in the application of the organic photorefractive material.
As a hologram recording material which does not require an external electric field, an organic material having an azobenzene skeleton is known. In hologram recording, the photoisomerization reaction of an azobenzene skeleton plays an important role. When a film using such a material is irradiated with linearly polarized light, the azobenzene skeleton is realigned due to the isomerization cycle of trans-cis-trans.
The azobenzene skeleton absorbs light corresponding to a p-p* transition, and is excited from a thermally stable trans form to a cis form to cause the modulation of a refractive index. The cis form generated by photoexcitation returns to a trans form spontaneously due to thermal relaxation. Therefore, when natural light is used as a light source, a large modulation of a refractive index cannot be expected. However, the following is known. When polarized light is used as a light source, only a trans form a zobenzene skeleton having an electric field matched with an electric vector of polarized light is excited, and further, when the excited cis form azobenzene skeleton returns to a trans form due to thermal relaxation, the excited cis form azobenzene skeleton returns to a trans form having an electric field perpendicular to the electric vector of polarized light. Thus, the azobenzene skeleton that cannot be excited is accumulated in the light-irradiated part to cause birefringence, and the modulation of a refractive index due to the birefringence is caused (Weigert effect: for example, see Next-generation polymer/supermolecule controlling light, NTS (2000) edited by The Society of Polymer Science, Japan) Further, when a light-irradiated site is heated or the like, the light-irradiated site changes to a trans form having random alignment (electric field) in an initial state to eliminate birefringence due to the molecular movement, whereby data can be written again. Due to the change in alignment, photoisomerization, i.e., birefringence and dichroism are induced, whereby hologram recording can be performed. Thus, an organic material having an azobenzene skeleton has a potential for a rewritable optical recording material, and in particular, for a hologram recording material.
The modulation of a refractive index of only an azo dye increases in proportion to the amount of the azo dye, so a material having large absorption is required for obtaining a large modulation of a refractive index, and a large output laser is required.
However, it has been reported that, when a liquid crystalline compound as well as an azo dye are used, large induction of birefringence is possible even with a relatively small absorption amount (for example, see Non-Patent Document 1). More specifically, for example, when a mixture of an azo dye and a liquid crystalline compound is used, liquid crystalline molecules are further aligned from the azo dye aligned by polarization, whereby extremely large birefringence can be expressed, and there is an effect of enhancing the modulation of a refractive index.
As such a hologram recording material, hologram recording materials using a polymer containing azobenzene, which have an azobenzene site and a liquid crystal site with a particular structure at a side chain and have an acrylate or methacrylate main chain, have been disclosed (for example, see Patent Documents 1 and 2). However, the materials are insufficient for an optical recording medium in both sensitivity (recording speed) and recording density.
Further, Non-Patent Document 1 describes a polymer compound suitable for hologram recording. However, in order to realize writing in a short period of time, it is necessary to induce anisotropy previously over the entire medium, and pre-treatment therefor is required. Further, multi-recording results of hologram using a recording medium having a thickness of 500 μm are described, but it takes 30 seconds for hologram recording, which cannot be considered to be sufficient as a practical recording speed.
Thus, a polymer material containing an azobenzene skeleton that has been reported is difficult to be used as a volume hologram material forming a plurality of holograms in an optical recording medium. More specifically, it is difficult to produce a thick film medium that achieves high-speed recording of digital data by realizing a high-diffraction efficiency, and a thickness of about 40 μm is a limit as a practical medium (for example, see Non-Patent Document 2).
Further, in the case of using liquid crystallinity, light scattering caused by liquid crystallinity occurs, so that the liquid crystallinity is difficult to be applied to an application using a transparent material. For example, a material can also be made transparent by uniformly aligning liquid crystal by a uniform alignment technique. However, uniform alignment can be performed only with a thickness of about several μm, so the liquid crystallinity is not suitable in the case of using a thick film.
Further, according to a method of using a liquid crystalline polymer, a polymer compound solution exhibiting thermotropic liquid crystallinity is applied to a substrate subjected to alignment treatment, and thereafter, the substrate is heat-treated at a temperature at which the liquid crystalline polymer exhibits liquid crystallinity to obtain desired alignment. After the alignment, the liquid crystalline polymer is kept in a glass state, whereby the alignment is immobilized. However, when the liquid crystalline polymer is applied to the substrate subjected to alignment treatment, the liquid crystalline polymer is applied while being dissolved in a solvent. Therefore, the liquid crystalline polymer cannot be applied to a substrate having low solvent resistance such as some kinds of plastic. Further, the liquid crystalline polymer has less compatibility with another component, so a synthetic operation such as copolymerization needs to be performed, for example, in order to combine functional sites of a liquid-crystal group, an azobenzene skeleton, and the like.
As a method of obtaining a liquid crystal compound excellent in compatibility, the idea of vitrified liquid crystal has been reported. A liquid crystal compound expressing vitrified liquid crystal has a plurality of liquid-crystal groups at ends, and the liquid-crystal groups and a core portion are connected via connecting groups. Due to such a structure, the enhancement of solubility and uniform application performance to some degree are recognized.
The uniform application performance is exhibited effectively in the case where a liquid crystal phase expressed by a liquid crystal material is only a nematic liquid crystal phase. Conversely, when a liquid crystal phase contains a phase close to crystal such as a smectic phase, uniform application becomes difficult due to the crystallization and smectic liquid crystallization during the application to a substrate and alignment treatment. Therefore, in vitrified liquid crystal, a liquid crystal compound having a lateral substituent is designed so as to express a single nematic liquid crystal phase by decreasing the crystallinity of liquid crystal, whereby uniform application performance is ensured. However, in a liquid crystal compound having a liquid-crystal group with a simple structure having no lateral substituent, a single nematic liquid crystal phase is not expressed, and the development of such a liquid crystal compound is desired.    Patent Document 1: JP 2000-514468 A    Patent Document 2: JP 2002-539476 A    Non-Patent Document 1: H. Ringsdorf and H-W. Schmidt, Makromol. Chem, 1327-1334 (1984)    Non-Patent Document 2: H. J. Coufal, D. Psaltis G. T. Sincerbox eds.: Holographic Data Storage, Springer, p. 222 (2000)